Add //rust/private/BUILD (therefore create a package there) (#569)

This PR achieves the following:

* buildifier bzl-visibility check was not detecting violations as it assumes private package, not just a directory
* adds //rust:common.bzl file where all the publicly available and supported API for writing custom rules interacting with Rust will reside
* the private package now sets the stability expectations, and it forces us to think about API layering and abstractions.

This turned out to be much bigger PR than expected :(